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P51 heads: How much more airflow with bigger valves?

8K views 14 replies 4 participants last post by  Big Block Notch 
#1 · (Edited)
I've been running the P51s in my Mustang for several years now and really like them. With some moderate porting, the intake ports are flowing 415-420 cfm right now with the standard 2.250-inch valves. How much airflow can I realistically pick up by stepping up to larger valves? I need to switch over to titanium intake valves anyway, so now would be the time to do it. The new combo will have a 4.600 bore, so I'll have some more real estate in there. Thanks!
 
#5 ·
You could use this for a compairison
http://users.erols.com/srweiss/calcafhp.htm

Real issue is the complete intake system. Will your intake actually flow more air, the carb.
An intake that flows 212 CFM bolted to a head that flows 235 CFM the intake will still flow 212 CFM bolted to a head that flows 360 CFM.
One shuld if possible flow the completer intake system at least with the intake
bolted to the head on the flow bench.
Flow testing an intake with a common plenum all ports but the one being flowed should be taped closed at the head gasket flange or the reading will be off (much higher).

Most setups I have dealt with the intake manifold is the bottle neck.
 
#6 · (Edited)
Good point, although my plans to put twin 88mm turbos on this combo probably throws a monkey wrench into the mix. Since I have to upgrade from stainless to titanium intake valves, I figured I might as well put a slightly larger diameter valve in it if it's worth some airflow. Realistically, all the additional airflow will be doing with a turbo combo is make the exact same power as it would with a smaller valve but at slightly lower (ie negligible) boost pressure.
 
#9 ·
I'm also not really trying to push the set up to its maximum potential. I'd just like to be able to make close to the 2000 whp range if desired but intend on a more conservative tune up for regular use. I have always had modular motors where the 4v heads from the facotry with minor adjustments is all you need to make 4 digit power. This is my first big block and I just don't want to shoot myself in the foot by picking the wrong heads. Of course you need to know what heads you will run when choosing pistons for final compression ratios etc. That's where I am at with my build right now.
 
#13 ·
The original question was in regards to the additional flow with bigger intake valves in P-51 heads. I answered that question and I do feel it is worth it,,,,,IF a guy is seeking additional power in a N/A P-51 headed engine.

However, now that the rest of the story has been revealed, I would not worry about adding additional intake flow, because that is going to be taken care of by the turbos and forced induction. If you are going with P-51 heads, then just stay with the 2.250" intake valves. The real bottleneck in the system will be with the exhaust ports flowing 215- 250 cfm, depending on whether they are ported or not. With any OEM style BB Ford head and turbos, the exhaust flow will be the problem area.

If other heads are available as an option, then by all means I would step up to any of the 18 bolt heads with turbos. You will have better head gasket clamping force with the 18 bolt heads. Those head options include the TFS A-460-18 bolt, the C-460, the Pro-Filer 205 and the Thor heads. Furthermore, your exhaust flow will be much improved with the TFS A-460-18 heads flowing about 325-330 cfm ported, and the C-460 and Pro-Filer flowing about 360 cfm ported, and the Thors flowing about 370 cfm ported.

The additonal exhaust flow will reduce your boost pressure,(all other factors being equal) because "the system" becomes less restrictive to flow.

Hope this helps,
 
#15 ·
Thanks for the great advice as always, Charlie. I was considering the Trick Flow's anyway, and didn't realize how much of an advantage in exhaust flow they have over the P51s.

My turbo headers have already been fabbed up, so I wonder if there's any hope of salvaging what I've got. I'm hoping I can re-flange them for the BBC style exhaust ports, but that's probably wishful thinking.
 
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